The Istanbul Bar Association filed a criminal complaint against electoral board head Sadi Guven for "wrongful conduct" and "altering the result of the election".

The High Electoral Board announced in a written statement its decision by a 10-1 vote to reject three requests by the opposition.

Hundreds of people queued and protested outside the Turkish election board's offices in Ankara and Istanbul to demand a controversial last-minute decision during Sunday's referendum to accept ballots without official stamps verifying them as genuine was overturned.

Turkey's main opposition party urged the country's electoral board Monday to cancel the results of a landmark referendum that granted sweeping new powers to Erdogan, citing what it called substantial voting irregularities.

A referendum broadening Erdogan's presidential powers narrowly passed on Sunday - a move that the country's opposition says will inch Turkey, a democracy, closer to authoritarianism. He added that "we expect the main opposition party's leader to act more responsibly".

Thousands took to the streets in Istanbul, Ankara and elsewhere in Turkey to protest the results of the referendum.

U.S. President Donald Trump calls Turkish counterpart Recep Tayyip Erdogan on April 17, 2017 to congratulate him on constitutional referendum "victory".

Erdogan has dismissed the criticism from the observer mission, telling the monitors that they should "know your place".

Worldwide election monitors, including from the Organization for Security and Cooperation in Europe, noted a series of irregularities, and said the decision to accept as valid ballots without official stamps undermined safeguards against fraud and was contrary to Turkish law.

"We will determine a date for the two presidents to meet in May before the North Atlantic Treaty Organisation summit as previously confirmed by the leaders", Foreign Minister Mevlut Cavusoglu tells reporters in Ankara.